Memory devices are typically provided as internal, semiconductor, integrated circuit devices in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), static RAM (SRAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory has developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage (Vt) of the memory cells, through programming (which is often referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data state (e.g., data value) of each memory cell. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
A NAND flash memory is a common type of flash memory device, so called for the logical form in which the basic memory cell configuration is arranged. Typically, the array of memory cells for NAND flash memory is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series between a pair of select gates, e.g., a source select transistor and a drain select transistor. Each source select transistor may be connected to a source, while each drain select transistor may be connected to a data line, such as column bit line. Variations using more than one select gate between a string of memory cells and the source, and/or between the string of memory cells and the data line, are known.
SRAM memory is often referred to as bistable as it may maintain one of two stable data state through the use of internal feedback as long as the memory cells receive power. SRAM memory tends to facilitate faster access, e.g., programming and reading, than flash memory. In addition, a data state of an SRAM memory cell can be changed without first erasing, as is often necessary with flash memory. Furthermore, SRAM memory is capable of maintaining its data state without requiring a refresh operation, as is often necessary with RAM memory.
Because of particular advantages, SRAM memory finds multiple uses. For example, cache memory for processors, disk drives and solid-state drives might utilize SRAM memory. In addition, due to their fast access and easy re-write, SRAM memory might be utilized for data logging in many vehicle subsystems, including infotainment systems, instrument cluster, engine control, driver assistance and black boxes. Although an SRAM memory cell does not require a refresh operation to maintain its data state, a loss of power in an uncontrolled manner, e.g., asynchronous power loss, will cause its data to be lost.